The hypothesis to be tested is that progressively decreasing physical activity, and, therefore, deconditioning is an important element in the aging process. In order to test this hypothesis animals will be allowed to run spontaneously in activity wheels and compared with sedentary control cage confined rats, with sedentary rats whose food is restricted so that their body weights are the same as running rats and with a group of animals forced to run in motor driven activity wheels at the same levels of exercise as those most active spontaneously running rats. The first series of experiments will be from 15 months to 25 months of age to determine if activity prevents deconditioning and aging effects. A separate group of forced runners will also be studied. Depending on the results of those studies, the experiment will be then extended to include a larger group of forced runners. Hearts will be studied in the isolated working rat heart apparatus. Measures of myocardial function and of maximal coronary flow will be included. Biochemical measurements will include contractile protein ATPase's, myosin heavy chain isoenzymes, and in selected animals function of sarcoplasmic reticulum. If the hypothesis is proven correct in rats, the implications are profound for continued cardiovascular health in humans.